Many of the pills marketed as safe herbal alternatives to Viagra and other prescription sex medications pose a hidden danger. For men on common heart and blood-pressure drugs, popping one could lead to a stroke, or even death.

"All-natural" products with names like Stamina-RX and Vigor-25 promise rare Asian ingredients but many work only because they contain unregulated versions of the prescription medications they are supposed to replace. That represents a danger to the millions of men who take nitrate-based drugs, prescribed for high blood pressure and to regulate heart disease. When mixed, nitrates and impotency pharmaceuticals can lower blood pressure catastrophically.

Public health officials don't know the extent of the problem but agree that incidents are vastly underreported. National tracking systems are believed to capture  as few as 1%  of the health problems caused by these "safe herbal alternatives", which have turned up in Thailand, Taiwan, Canada, Australia, New Zealand, Hong Kong, Malaysia, the United Kingdom and the United States. Testing undertaken by Pfizer, the company that developed Viagra, showed that 69% of 3,400 supplements it purchased in China contained the active ingredient in Viagra.